The use of penetration devices to obtain information relating to the mechanical characteristics of a material being tested is very well known. Typically, an indenter is forced into a sample of a material and then extracted therefrom, leaving an indentation in the surface of the sample. The depth of penetration, surface or projected area of the indentation and the force applied to the indenter provide information indicative of material properties such as hardness or modulus of elasticity.
The modification of surface layers and the use of surface coatings to provide improved chemical, mechanical, electrical or decorative properties is rapidly increasing. Such modifications include very thin surface treatments such as vacuum deposited coatings, ion-implanted surface layers and surface hardening. In order that the properties of such coatings and layers be better understood, testing apparatus has been developed to carry out ultra-microhardness testing.
Ultra-microindentation is a technique for obtaining information about the mechanical properties of a surface material from relationships between the depth of penetration of an indenter and resistance to its penetration. Ultra-microindentations are produced by pressing the indenter, which is usually a diamond pyramid, into the surface under the control of an ultra-microindenting system (UMIS).
To ensure that the probe of a UMIS does not cause cracking in the sample coating or surface layer and also to ensure that the characteristics of the coating are tested independently of the substrate characteristics, it has been recommended that the depth of penetration should be small in relation to the total layer or coating thickness. As, for example, the depth of penetration of implanted ions typically does not exceed 1 .mu.m, it is desirable to be able to carry out microhardness testing using penetrations as little as 0.1 .mu.m. If an indenter with Vickers type geometry is used to produce an indentation of 0.1 .mu.m depth, the diagonal width of the indentation will be of the order of 0.7 .mu.m.
Measurement of impressions having widths less than about 1 .mu.m are impossible to perform without the aid of an electron microscope. Skilled addressees will be aware that electron microscopes are extremely expensive, both in initial purchase cost and operating cost. Consequently, there is a need for a penetration measuring instrument capable of use in ultra-microhardness testing which can provide data indicative of the depth of indentation without the need to perform further measurement. As an indenter will be of known geometry the width surface or projected area of an indenter can be defined if the depth of indentation is known. It has been shown that if a device can provide information indicative of the indenter position and the indenting force at intervals during the loading and unloading phases, the data thus generated is sufficient to allow deductions to be made about the dynamic and static resistance to abrasion, to local penetration and about the static and dynamic elastic properties of the new surface material. That is, it is possible to carry out the appropriate calculations of material properties without recourse to measurement by a microscope of any kind, be it optical, electron or otherwise.
In order for such a penetration measuring apparatus to be successful, the indenter motion should be produced by an actuator which is capable of producing smooth, vibration free motion at very low speed. The effects of any inertial forces which, for example, can arise from dead weight loading, must be avoided as far as possible. In the present invention the solution is to support the indenter by elastic coupling means on a carriage assembly and, as the carriage assembly is moved an indenting force is transferred to the probe. In this way the deflection of the elastic coupling means is always just sufficient to balance the instantaneous resistance to penetration and, provided the indenting mechanism has low mass, and moves slowly, inertial forces are minimal.